build: Move python detection back into waf (instead of in configure and Makefile)

This avoids creating a mini-configure in the configure script.

Users wishing to use python2 to build need to specify PYTHON=
to both ./configure and make

After we merged the python3 change, it became clear that relying on systems prefixing
the correct python just causes trouble and make debugging harder, so only use $PYTHON
for the override, not the default case